Alkali ion disorder in (Na,Li)2FeTi(PO4)3and Na2FeZr(PO4)3NASICON phases by neutron diffraction

NASICON materials are crystalline phosphates of transition metals Me and alkali atoms A, according to the formula A x M y (PO 4 ) 3 , which display a high mobility of the A + ion, and are thus promising materials for applications as solid state electrolytes and electrodes in microbatteries.The structural characterization of the mobile ion environment, and of its bonding interactions with the neighbouring atoms, is essential to understand the mechanisms of ionic conduction in these phases.We have been investigating a number of NASICON compounds by high-resolution neutron powder diffraction at variable temperature (HRPD time-of-flight diffractometer at the ISIS Facility, Rutherford Appleton Laboratory, U.K.; resolution Dd/d » 4x10 -4 ).By use of Rietveld refinements coupled with difference Fourier maps, multiple disorder effects involving the alkali ions in the large M1 and M2 cavities of the NASICON framework can be detected [1,2].The structures of the Na 2 FeTi(PO 4 ) 3 and Na 2 FeZr(PO 4 ) 3 phases are presented here, together with preliminary results for the partially lithium-exchanged FeTi compound.The two sodium materials were synthesized by solid state reactions.Rietveld refinements converged to R p factors of 0.068 and 0.094 (space group R-3c, Z=6) for the two above phases, respectively.In the Na 2 FeTi(PO 4 ) 3 case (a=8.59665(3),c=21.7137(1)Å), half of Na (Na1) was located within the M1 and half (Na2) within the M2 cavity; however, about 40% of Na1 lies exactly on the (0,0,0) site with -3 symmetry, but the remaining part is disordered in an off-centre position.Further, also Na2 is slightly displaced from the centre of the M2 hollow at (2/3,0,1/4), attaining a disordered configuration.Such results are at variance with the fully ordered structural model previously reported for the same compound [3].A different situation was observed in Na 2 FeZr(PO 4 ) 3 (a=8.76828(4),c=22.2706(2)Å), where only 0.64 atoms per formula unit lie on the M1 site, and 1.36 atoms per f.u. are distributed between the M2 site and a disordered position also located within the M2 cavity but significantly off-centre.Lithium exchange was performed in water solution on Na 2 FeTi(PO 4 ) 3 , and the product was studied by neutron diffraction.Occupancy refinements and difference Fourier syntheses indicate that about 50% of sodium was actually exchanged.Preliminary results of the structural analysis show that the symmetry lowers from R-3c to R-3, consistent with what reported in [3]; sodium occupies the M1 sites in an orderly way, whereas lithium goes into the M2 cavity on a disordered position similar to that observed in Li 1.5 Fe 0.5 Ti 1.5 (PO 4 ) 3 [2].Models of ionic mobility and diffusion pathways can be proposed on the basis of the disorder found.s12.m34.p12Ferroelectric Behavior in Perovskite Ceramics of Composition Ba 1-x Y 2x/3 Ti×O 3 .